The present invention relates to an ultrasonic sensor for use in detecting objects. More specifically, the ultrasonic sensor is utilized to detect both presence and range of objects in very close proximity to the sensor itself. Further, the sensor of the present invention can be used to detect objects immediately adjacent to the sensing surface of the sensor housing.
In automated industrial applications, sensors are used for numerous purposes. These purposes can include object detection during an automated manufacturing process, range detection during quality assurance checks, etc. In these industrial applications, it is necessary for these sensors to be very rugged, efficient and environmentally robust as the environments of use can be fairly harsh; the processes can be very dirty with particles of all types in the area of the sensor.
A common device for sensing the presence of objects is the use of eddy current killed oscillators (ECKOs). These devices have two shortcomings: (1) they can sense only metallic targets, and (2) they have very limited range relative to their physical size. For example, a maximum range of 1/3 the diameter of the sensor is common--a maximum range equal to the diameter of the sensor is achievable, however very unusual. Therefore, to perform sensing at any reasonable range would require a very large sensor (e.g. a range up to two (2) inches would require a sensor with a diameter of at least two (2) inches).
Ultrasonic sensing systems provide a much more efficient and effective method of longer range detection. These sensors require the use of a transducer to produce ultrasonic signals. These signals are propagated through a sensing medium and the same transducer can be used to detect returning signals. In most applications, the sensing medium is simply air. Several transducers are capable of accomplishing this function including piezoelectric polymer films such a polyvinylidene fluoride (PVF.sub.2), and electrostatic transducers (such as those presently offered and used by the Polaroid Corporation).
Piezoceramic ultrasonic transducers are the transducers of choice for rugged, industrial applications because they are efficient and environmentally robust. These sensors have been used in industry for numerous applications; however have not been capable of short range object detection until recently. This is due to the inherent characteristics of present ultrasonic sensing systems.
Prior art piezoceramic ultrasonic transducers have been designed to have very good long range detection capabilities. In designing these sensors, it was always necessary to maximize the Q value of the transducer and minimize the frequency of operation. High Q amplifies the returning signal, and low frequency serves to reduce the attenuation of ultrasound in air because attenuation is a function of frequency. These prior art sensors, however, do not display the short range sensitivity required by the present invention.
It is well known by those skilled in the art that piezoceramic transducers have a ringing characteristic or resonant characteristic. More specifically, when the energizing signal for the transducer is removed, the device continues to resonate, albeit at a continuously decreasing amplitude, due to the resonant nature of the transducer itself. This ringing characteristic eliminates the ability to use ultrasonic transducers at very short ranges.
In operation, when the transducer is used in a sensing system, an electronic signal is provided to energize the transducer and consequently produce the ultrasonic signal. This energy is provided in a burst such that excitation is removed from the transducer at a point in time. When excitation is removed from the transducer, it is incapable of sensing any return signals created until the residual resonance (ringing) is damped below a level that returning signals would produce. As the objects being sensed creates reflections or returning signals, it is essential that the sensor be able to detect these signals. As is also well known in the art, when ultrasonic signals are transmitted through air their amplitude attenuates. In designing a sensing system using ultrasonic signals, it is therefore necessary to account for this attenuation when designing the sensitivity of the transducers (i.e. the system must be capable of detecting these attenuated return signals).
The operating range of an ultrasonic sensor is greatly affected by its ringing or resonating characteristics. When designing for close range sensitivity, the ring out time or standoff time (i.e. time required for residual resonance to degrade to an acceptable level) is directly determinative of short range operability. As will be further understood, a sensor must allow for this ringing to reach acceptable levels before the transducer is capable of sensing attenuated return signals. Consequently, short range detection has not been possible using present piezoceramic transducers.